Conventionally, volatile memory elements, such as DRAM and SRAM, using silicon substrates have been widely used. In the future, however rewritable and nonvolatile memory elements are expected to be developed. Nevertheless, conventional manufacture of memory elements using silicon substrates have required photolithography, which is such a troublesome and expensive process that there has been a problem that memory elements as end products become too expensive. As alternatives to such conventional memory elements using silicon substrates, some nonvolatile memory elements using organic materials are proposed.
For example, as memory elements using organic low molecular compounds capable of yielding an abrupt change in electric conductivity in response to a change in applied voltage, there are proposed memory elements using 2-amino-4,5-imidazolecarbonitrile (Applied Physics Letters, by L. Ma et al., 82(9), p 1419-1421 (2003)) or those using 7,7,8,8-tetracyanoquinodimethane copper complex (Applied Physics Letters, by R. S. Potember et al., 34(6), p 405-407 (2003)).
Those memory elements using organic low molecular compounds include a conducting layer such as that of metallic aluminum in an organic layer or between an electrode layer and an organic layer. By applying a voltage not less than a threshold voltage, they change from a high-resistivity state to a low-resistivity state to thereby keep the low-resistivity state permanently. When a reverse voltage is applied to those elements in such low-resistivity state, they will return to the high-resistivity state at a given threshold.
Although the mechanism of the change in resistivity by the application of voltage still remains to be explained, it may be attributed to some electrical interface phenomenon caused by space charges accumulated on an interface between the conducive layer and the organic layer or between the organic layer and the electrode layer provided in the interior of the memory element.
According to the memory elements using such organic low molecular compounds, however, the manufacturing process thereof is complicated, and the production yield of the memory elements is not high, as they are produced by forming a thin film of the low molecular compounds through the processes requiring ultrahigh vacuum condition, such as vacuum deposition method, electron beam method, sputtering method, etc.
On the other hand, some attempts to manufacture memory elements using organic high polymer compounds have been made. For example, there is proposed a single-layer-type memory element in which the mixture (PEDOT:PSS) of poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) is provided between an anode comprised of indium tin oxide (ITO) and a cathode comprised of aluminum (by S. Moller et al, Nature, 426, p 166-169 (2003)).
Whilst the memory elements obtained by using such organic high polymer compounds change from a low-resistivity state to a high-resistivity state by applying a voltage not less than a threshold voltage, yet an on-off ratio thereof was not so high. In addition, as PEDOT:PSS is water-soluble polymer, there has been a problem that formation of a uniform layer of PEDOT:PSS is difficult, resulting in difficulties in forming multilayer memory elements.
Further, although various types of nonvolatile memory elements have been developed until now, there are different problems involved in any type thereof. Flash memories have already been come into practical use, but they operate slower than DRAM or SRAM. Besides, they have a limitation in write cycle life. In an attempt to overcome the problems, various types of elements such as FeRAM, SONOS/NROM, MRAM and the like have been developed, yet any of them needs the vacuum deposition process at the time of manufacturing, since they employ ceramic materials such as metal oxides and sulfides, thus leading to high manufacturing cost. To meet a demand to manufacture flexible and large-area elements at low cost, it is imperative that materials such as organic polymers as well as an inexpensive wet process such as a spin-coating process be employed.
Under the circumstances, TFE (Thin Film Electronics) AB proposed, jointly with Intel Corporation, a certain polymer memory using ferroelectric polymers such as homopolymer of vinylidene fluoride or copolymer of trifluorovinylidene and trifluoroethylene (U.S. Pat. Nos. 6,055,180, 6,326,936; US Patent Application Publication No. 2003/0017623A1). One of the problems of the element is in that it is not able to perform nondestructive readout after writing. To solve the problem, Xerox Corporation developed a nonvolatile memory which makes both writing and reading repeatable, using a multilayered element comprised of similar polymer material and organic semiconductor (Japanese Unexamined Patent Application Publication No. 2004-040094). All of them, however, use a fluorinated polymer, thus leading to high cost in terms of both material and process.
On the other hand, AMD Inc. in the United States proposed, using a basic technique of Coatue Corp, a venture company of UCLA, a memory element adopting a new principle different from that of the foregoing ferroelectric polymer (U.S. patent Application Publication Nos. 2003/0155602A1, 2002/0163828A1; WO 02091385). That is to say, the element has a structure in which pi-conjugated polymer (for example, polyacetylene or polyaniline or the like), doped with ionic-dissociation salt such as sodium chloride or cesium chloride, is sandwiched between two metal electrodes. When a voltage is applied to a film of this polymer, then the doping salt thus added is dissociated into plus and minus ions, which are then drawn toward opposite-polarity electrodes, respectively, so that the electric conductivity of the film changes reversibly. Taking advantage of this phenomenon, the switching function thereof is developed. Nevertheless, the reading of this memory is destructive, so the memory is used as a write-only type. In addition, the switching speed is as slow as in msec order. It is attributed to the transfer of relatively large ions in the solid film of the pi-conjugated polymer composed of hard molecular chains.
Although not the ones using the foregoing-mentioned polymer materials, other types of memory elements using low molecule organic materials also have been studied by many groups in the past. As a pioneering example, R. S. Potember et al. (Johns Hopkins University) proposed a memory element which has such a structure that tetracyanoquinodimethane (TCNQ) thin film is sandwiched between two copper electrodes (U.S. Pat. No. 4,371,883; Appl. Phys. Lett., ((1979), 34(6), 405). TCNQ is known to be a compound having strong electron acceptability, allowing electrons to transfer to and from copper, thus forming a sort of charge-transfer type complex (CT complex). It is considered that a layer of such CT complex is formed to an about 10 micrometer thickness on an interface between TCNQ and the copper electrodes, thus allowing the switching function to appear. The switching speed of this element is extremely fast, as fast as 10 nanoseconds, and besides, on-off ratio is 10,000 or above, and therefore, it attracted attention as a promising memory element. This initiated the active studies of the application of CT complex to a switch/memory element. Particularly in Japan, Gunji SAITO et al. took the lead in the studies of various types of CT complexes in the TCNQ system (Appl. Phys. Lett., (1989), 55(20), 2111). Some companies also filed several patent applications in this connection (For example, Japanese Unexamined Patent Application Publication Nos. 62-095882 and 62-095883 filed by Canon Inc.: Japanese Unexamined Patent Application Publication Nos. 03-137894 and 03-137896 by Matsushita Electric Works, Ltd.). Although there are varying explanations about the mechanism thereof, a likely explanation is that electric conductivity of the film changes because of the transfer of CT complex from a crystalline phase to an amorphous phase due to the generation of Joule heat associated with applied voltage. In any case, films having evaporated organic low molecular crystals involve many technical difficulties in obtaining uniform films, and there remains a problem to be solved in terms of production cost as well.
There was studied another example of a memory element that uses copper electrodes like in the foregoing-mentioned example yet has a different mechanism from that of Potember et al. It has long been known that applying a voltage to two metal electrodes (of gold, copper or the like) with a thin film of organic polymer, metal oxide or sulfide being sandwiched therebetween, causes the metal used as electrodes to ionize in an electrical field and transfer in the film. It was J. G. Simmons and R. R. Verderber who showed the theoretical feasibility of application of this phenomenon to a switch element (Proc. Roy. Soc., (1967), A301, 77). It is believed that copper ions thus produced are allowed to transfer toward the counter electrodes and then reduced to metal copper, and then finally reach them after growing in a filamentary structure in the film, thus allowing electrically conductive paths to be formed therein. Taking advantage of such acting mechanism, NEC Corporation proposed a certain novel switch element (Appl. Phys. Lett., (2003), 82(18), 3032). On the other hand, a group of Y. Yang et al. attributed what makes contribution to such electrical conductivity not to metal copper, but to copper ions (Appl. Phys. Lett., (2004), 84(24), 4908). Since the used materials are different, discussion on the same premise is impossible. Anyway, it is attracting attention that such elements do have switching function or memory function. It should be noted, however, that this type of elements also has a problem that switching speed is rather slow (microsecond order).
Further, Y. Yang et al. (University of California) used as a memory an element having a five-layer structure in which an imidazole-based compound of low molecular weight is sandwiched between two aluminum electrodes, and another aluminum layer is placed in an intermediate layer thus formed (US Patent Application Publication No. 2004/0,027,849A1; Appl. Phys. Lett., (2002)), 80(16), 2997). Here, it is believed that the intermediate aluminum layer acts as if a floating gate electrode of a flash memory. In other words, space charges are stored therein, so that the memory function appears due to the charges coming in and out therefrom. On the other hand, J. C. Scott et al. (IBM in the United States) examined another element as a memory, said element having such a structure that nanoparticles of metals (e.g., gold or the like) are homogeneously dispersed in an organic layer instead of providing the foregoing-mentioned intermediate aluminum layer (Appl. Phys. Lett., (2004), 84,607). As for terminal electrodes, they used aluminum electrodes as Yang et al did so. They explained that what is important for the stable driving of the element is not the form of the metal (whether in the form of thin film or in particles) in the organic layer, but the formation of an ultrathin oxide layer on a surface of the aluminum terminal electrode contacting the organic layer. A group of Fuji Electric Co., Ltd, however, reported that the same memory characteristic appears event though the organic layer contains no metal. (by H. Kawakami et al., Proc. SPIE (2003), vol. 5217, 71). Thus, the operative mechanism of that kind of elements still needs careful examination.
As explained above, there is no perfect key approach proposed at this time yet to developing materials and process so as to manufacture a flexible, large-area and rewritable memory element at low cost. There is not any practically satisfactory solution yet in terms of basic characteristic, such as switching speed, writing/reading repeatability, memory retention time, etc, as well as production cost.